The invention relates to a pulse generating circuit for generating pulses in synchronism with horizontal synchronizing signals included in a video signal in a computer graphics (CG) system or computer-aided design (CAD) system.
A video signal in a CG system or CAD system has a resolution higher than that in an usual NTSC or PAL television system. Therefore, a high definition image can be obtained by printing an image in accordance with the video signal on a sheet of paper.
In order to print the image in accordance with such a video signal on a sheet of paper, the video signal is needed to be memorized in a memory. And, for writing on and reading from the memory the video signal, it is necessary to obtain pulses which are synchronized with horizontal synchronizing signals included in the video signal and have a frequency higher than the horizontal synchronizing signals.
A PLL circuit is employed to produce the pulses.
However, there exist no horizontal synchronizing signals in a vertical interval of the video signal in CG system or CAD system. Even if there exist signals in the vertical interval, the signals are not synchronized with the horizontal synchronizing signals. Therefore, the operation of PLL circuit is disturbed in the vertical interval and it is difficult to produce stably pulses in synchronism with the horizontal synchronizing signals.